How to calculate/update values for a nested list in python?

I'm really stuck at a problem, where I completely don't understand why the output is what it is and not what I expected. I basically want to calculate values for the elements of a nested list which is initialized with only zeros. Here is the code of a simple form of my problem:

aa = [[0] * 2] * 2

for i in range(2):
    for j in range(2):
        for k in range(0, 10):
            aa[i][j] += k

I expected here the output for "aa" to be: [[45, 45], [45, 45]].

But the output is: [[90, 90], [90, 90]]

How can that be? Any explanation would be helpful. After debugging I saw that Python adds the k always to both lists in the list, although i is for example currently 0.

Solution

That is because the shallow copy of list. aa = [[0] * 2] * 2 creates shallow copy, try aa = [[0]*2 for i in range(2)]

aa = [[0]*2]*2

print(aa[0]) #[0,0]
print(aa[1]) # [0,0]

aa[0][0] = 2
aa[0][1] = 2

print(aa[0]) #[2,2]
print(aa[1])  # [2,2] !!

# +-------+-------+
# | aa[0] | aa[1] |
# +-------+-------+
#   |         |
#   |         |
#    \       /
#     [0 , 0]
    
#     Both referring to the same list
#     So whatever operation you do in aa[0] will reflect in aa[1] and vice versa
#     Because effectively they are pointing to same list
#     id(aa[0]) == id(aa[1]) ---> True

aa = [[0]*2 for i in range(2)]

for i in range(2):
    for j in range(2):
        for k in range(0, 10):
            aa[i][j] += k

[[45, 45], [45, 45]]